1. Field of the Invention
The present invention relates to power control in a mobile communications system, specifically, to a method of controlling forward power of a mobile communications system, in which forward power transmitted from a base station to a mobile is controlled using an erasure indicator bit of a reverse traffic channel frame.
2. Discussion of Related Art
In general, a CDMA system employs forward (from a base station to mobiles) power control and reverse (from mobiles to a base station) power control in order to maximize the number of users who can call in a specific bandwidth simultaneously and to obtain a predetermined amount of speech capacity, high speech quality and other advantages. The system capacity is maximized when the transmission power of each mobile is controlled to reach a base station, having a minimum signal-interference ratio. The transmission power of the mobile is controlled so that transmission signals of all mobiles in a zone are received by the base station""s receiver in nominal intensity, thereby allowing more mobile to be able to communicate with the base station with higher speech quality. When the mobile""s transmission power received by the base station is too low, bit error rate becomes very high. Thus, high speech quality cannot be obtained. On the contrary, when the reception power at the base station is too high, though the speech quality of the mobile becomes higher, interference to other mobiles using the same channel is increased. This deteriorates the speech quality unless the number of the entire subscribers is reduced.
The base station controls the forward power, corresponding to power measured at the mobiles, to support forward power control. The forward power control is for decreasing loss power in a mobile which is not busy or relatively close to the base station or less affected by multipath fading and radio wave shading effect or hardly receives interference of other base stations. For mobiles which are located in an area with difficult in reception or in the distance so as to have high error rate, additional power support is provided. The forward power control in an 800 MHz CDMA system uses a power measurement report message (PMRM). When the system informs a mobile of when the PMRM will be transmitted by selecting one of three operation modes (1;transmit the PMRM when the number of bad frames received by the mobile exceed a critical value, 2;transmit the PMRM periodically, 3;transmit the PMRM periodically even when the number of bad frames exceed the critical value), the mobile transmits the PMRM according to the selected operation mode. Then, a control station receives the PMRM to control the power, and informs the base station of it. The base station transmits signals to the mobile with the corrected power.
In the aforementioned conventional power control method, it takes considerable time for the mobile to transmit the PMRM. Accordingly, it is difficult to control the power rapidly in circumstances of sudden change of propagation environment. The critical value can be set to allow the mobile to transmit the PMRM unconditionally when the bad frame occurs. However, since the PMRM is a message, when the mobile receives the bad frame, it should inform the control station immediately through a method a method other than messaging in order to control the power rapidly.
U.S. Pat. No. 5,629,934 discloses a power control method using three power control bits. J-STD-008 which is a part of the standard of personal communication system (PCS) using CDMA includes a method of controlling forward channel power using PMRM, in which an erasure indicator (referred to as EI hereinafter) bit is put in the reverse frame of multiplex option 2 (referring to FIG. 4), and, when the mobile receives the bad frame, frames are transmitted, setting the EI bit located two frames behind to 1. However, the J-STD-008 does not contain how the EI bit is used.
As described above, in the conventional forward power control method, rapid power control cannot be performed in the circumstances of sudden change of propagation environment because it takes considerable time for the mobile to transmit the PMRM. Furthermore, though the critical value can be set to allow the mobile to transmit the PMRM unconditionally when the bad frame occurs, it takes time to use power control routine in call processing after pasing because the PMRM is a message. Moreover, when the bad frame occurs continuously, the control station""s traffic channel processor which deals with eight mobiles simultaneously is difficult to operate.
Accordingly, the present invention is directed to a method of controlling forward power of a mobile communications system that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method of controlling forward power of a mobile communications system, in which a control station controls the forward power using EI bit of reverse traffic channel frame.
Another object of the present invention is to provide a method of controlling forward power of a mobile communications system, which stabilizes calls in a rapidly changing propagation environment or hand off condition where a mobile moves rapidly.
A still another object of the present invention is to provide a method of controlling forward power of a mobile communications system, in which power control information is transmitted to the base station using the EI bit of reverse traffic channel frame, to allow the control station to control the forward power.
To accomplish the objects of the present invention, there is provided a method of controlling forward power of a mobile communications system comprising the steps of searching for EI bit of reverse traffic channel frame transmitted from a mobile, controlling forward power according to a transmission rate previously stored when it is judged that forward power control is required from the result of searching for EI bit, and searching for EI bit of reverse traffic channel frame after the controlling of forward power and varying the transmission rate according to the result of searching.
The mobile indicates information on forward power in the EI bit of reverse traffic channel frame. The control station searches for this EI bit, to judge whether forward power control is required or not. When it is judged that forward power control is required from the result of searching, the control station controls the forward power with large power increasing value BIG-UP and large power decreasing value BIG-DOWN when transmission is made at a full rate, and controls it with small power increasing value SMALL-UP and small power decreasing value SMALL-DOWN when transmission is carried out at a rate of below the full rate.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.